The present invention relates to a method of controlling a wire-cut electric discharge machine to cut a workpiece thereon along an arc or a corner with increased accuracy.
In particular the present invention relates to wire-cut electric discharge machines having a pair of upper and lower wire guides and a wire electrode extending therebetween and kept taut for applying a voltage across a gap between the wire electrode and a workpiece to produce spark discharge across the gap, thereby cutting the workpiece with discharge energy. The workpiece can be cut to a desired contour by moving the workpiece with respect to the wire electrode under cutting command data. As shown in FIG. 1, when the wire electrode 1 moves in and along a slot 3 in a given direction while cutting off the workpiece 2, a pressure is developed between the wire electrode 1 and the workpiece 2 due to the electric discharge to push back the wire electrode 1 in the direction of the arrows which is opposite to the direction in which the wire electrode 1 moves along, as shown in FIG. 2. The wire electrode 1 is therefore backed off or flexes from the position of the wire guides 4, 4. The cutting acuracy is not adversely affected to an appreciable extent by the amount of such wire flexing as long as the wire electrode 1 cuts the workpiece 2 along a rectilinear slot. However, when the workpiece changes its direction of relative movement through, for example, a right angle under a cutting command to cut a corner as shown in FIG. 3, the wire electrode has a tendency to be dragged inwardly of the corner due to the flexing of the wire electrode at a position in which the electric discharge takes place, with the result that the contour of the slot 3 as it is cut becomes different from a commanded shape as shown by the solid lines and has its configuration 3a rendered blunt or beveled as shown by the dotted lines. A similar error is also caused when cutting the workpiece along an arc as shown in FIG. 4.
It is known that the blunt shapes of such arcuate or angular corners cut in workpieces can be improved by changing discharge conditions. However, there has not yet been developed and hence there has been a need for a method of effecting optimum corner cutting control under various cutting conditions.